How buffer overflow in FuzzIxml.c sprintf() happens in C and how to fix it

How Buffer Overflow in FuzzIxml.c sprintf() Happens in C and How to Fix It

Summary

A critical buffer overflow vulnerability was discovered in fuzzer/FuzzIxml.c where sprintf() wrote a PID-formatted filename into a fixed 256-byte stack buffer without any bounds checking. The fix replaces sprintf() with snprintf(), explicitly passing the buffer size to prevent any overflow. While exploitation in this specific fuzzer context requires local access, the pattern is a textbook example of CWE-120 that developers should recognize and eliminate everywhere it appears.

Introduction

The fuzzer/FuzzIxml.c file is a libFuzzer harness — a small C program whose entire purpose is to stress-test the ixml library with malformed input. Because fuzzer harnesses run continuously, often under automation, and frequently with elevated privileges in CI/CD pipelines, a memory-safety bug here can have real consequences. A flaw at line 45 in the LLVMFuzzerTestOneInput function created a classic stack buffer overflow: sprintf() was used to construct a temporary filename from the process ID, with no mechanism to prevent the formatted output from exceeding the destination buffer's 256-byte capacity.

This post walks through exactly what went wrong, why it matters even in "internal" tooling, and how the one-line fix permanently closes the vulnerability.

The Vulnerability Explained

What the code was doing

Inside LLVMFuzzerTestOneInput, the harness needs a temporary file to write fuzzer-supplied bytes before passing them to the ixml parser. It constructs a filename in /tmp using the process ID to avoid collisions:

// Vulnerable code — fuzzer/FuzzIxml.c, line 45 (before fix)
char filename[256];
sprintf(filename, "/tmp/libfuzzer.%d", getpid());
fp = fopen(filename, "wb");

The buffer filename is declared on the stack with a hard-coded size of 256 bytes. The format string "/tmp/libfuzzer.%d" is 18 characters plus a variable-length decimal representation of the PID.

Why sprintf() is the problem

sprintf() has this signature:

int sprintf(char *str, const char *format, ...);

Notice what is absent: there is no parameter for the maximum number of bytes to write. sprintf() will write as many bytes as the formatted result requires, regardless of how large str actually is. If the formatted output exceeds sizeof(filename), bytes are written past the end of the stack buffer — overwriting the saved frame pointer, return address, or other local variables.

Is a PID really that dangerous?

On Linux, PIDs are typically 32-bit integers, so the maximum decimal representation is 10 digits (/tmp/libfuzzer.4294967295 = 26 bytes). Under normal conditions this fits comfortably in 256 bytes.

However, the vulnerability is real for several reasons:

1. Future format string changes: The format string is a maintenance target. A developer adding a hostname, timestamp, or user-supplied suffix to avoid collisions could push the length past 256 bytes without realizing the underlying sprintf() call has no guard.
2. Environment manipulation: An attacker with local access who can influence how the process is launched (e.g., via a wrapper script or LD_PRELOAD manipulation in some environments) may be able to influence related state.
3. Fuzzer infrastructure context: Fuzzers often run as part of automated pipelines. A compromised fuzzer binary in a CI/CD environment is a meaningful attack surface.
4. Code review and audit signal: The presence of sprintf() writing to a fixed buffer is a red flag that static analyzers, auditors, and future contributors will flag — creating noise and eroding trust in the codebase's security posture.

Attack scenario specific to this code

Consider a scenario where the fuzzer harness is extended to include a user-supplied seed corpus path in the filename:

// Hypothetical future change that makes the existing sprintf() dangerous
sprintf(filename, "/tmp/libfuzzer.%d.%s", getpid(), seed_corpus_dir);

If seed_corpus_dir is sourced from a command-line argument (which is exactly how libFuzzer harnesses are invoked), an attacker passing a long path string would overflow filename on the stack. The original sprintf() call, with no size guard, would silently allow this. The snprintf() fix prevents it unconditionally.

The Fix

The fix is a single-line change in fuzzer/FuzzIxml.c:

- sprintf(filename, "/tmp/libfuzzer.%d", getpid());
+ snprintf(filename, sizeof(filename), "/tmp/libfuzzer.%d", getpid());

Before

char filename[256];
sprintf(filename, "/tmp/libfuzzer.%d", getpid());

After

char filename[256];
snprintf(filename, sizeof(filename), "/tmp/libfuzzer.%d", getpid());

Why this fix works

snprintf() has this signature:

int snprintf(char *str, size_t size, const char *format, ...);

The second argument size is the maximum number of bytes (including the null terminator) that snprintf() is permitted to write. By passing sizeof(filename) — which evaluates to 256 at compile time — the call is guaranteed never to write beyond the buffer boundary.

Using sizeof(filename) rather than a hard-coded 256 is the idiomatic and safer choice: if the buffer declaration is ever changed (e.g., to char filename[512]), the snprintf() call automatically picks up the new size without requiring a separate edit.

What happens if the string would overflow?

snprintf() truncates the output to fit within size - 1 bytes and always null-terminates the result (as long as size > 0). The return value indicates how many bytes would have been written if the buffer were large enough — a pattern that allows callers to detect and handle truncation if needed:

int written = snprintf(filename, sizeof(filename), "/tmp/libfuzzer.%d", getpid());
if (written < 0 || (size_t)written >= sizeof(filename)) {
    // Handle truncation — the filename was cut short
    return 1;
}

The PR fix applies the core protection; adding explicit truncation detection is a further hardening step teams can adopt.

Prevention & Best Practices

1. Ban sprintf() in your codebase

The simplest prevention is a blanket policy: never use sprintf(). Replace it with snprintf() everywhere. Most modern C style guides (including the Linux kernel coding style and CERT C) explicitly prohibit sprintf() for this reason.

You can enforce this with a Semgrep rule:

rules:
  - id: no-sprintf
    patterns:
      - pattern: sprintf(...)
    message: "Use snprintf() with an explicit buffer size instead of sprintf()"
    languages: [c, cpp]
    severity: ERROR

2. Use sizeof() for buffer sizes, not magic numbers

When calling snprintf(), always pass sizeof(buffer) rather than a literal integer. This ensures the size argument stays in sync with the buffer declaration automatically.

// Fragile — size and buffer can diverge during refactoring
char buf[128];
snprintf(buf, 128, "...");

// Robust — size is always correct
char buf[128];
snprintf(buf, sizeof(buf), "...");

3. Enable compiler warnings and sanitizers

Modern GCC and Clang can detect many sprintf() overflow patterns at compile time:

gcc -Wall -Wformat -Wformat-overflow -fsanitize=address,undefined fuzzer/FuzzIxml.c

AddressSanitizer (ASan) will catch buffer overflows at runtime during fuzzing runs — making the fuzzer harness itself a detector of this class of bug.

4. Use static analysis in CI

Tools that detect this pattern automatically include:

• Semgrep with the c.lang.security.insecure-use-sprintf-fn rule
• Clang Static Analyzer (scan-build)
• Cppcheck (--enable=warning)
• Coverity (commercial)
• Orbis AppSec (detected this exact instance)

5. Consider safer string libraries for new code

For new C projects, consider string libraries that make buffer sizes explicit by design:

• strsafe.h (Windows)
• libbsd's strlcpy/strlcat
• std::string if C++ is an option

Security standards references

• CWE-120: Buffer Copy without Checking Size of Input
• CWE-121: Stack-based Buffer Overflow
• CERT C Rule STR07-C: Use the bounds-checking interfaces for string manipulation
• OWASP: Buffer Overflow

Key Takeaways

• sprintf() into a fixed-size stack buffer in FuzzIxml.c was a ticking clock: while the current format string was safe, any future extension of the filename pattern could have triggered a real overflow without any compiler warning.
• The fix is always the same for this pattern: replace sprintf(buf, fmt, ...) with snprintf(buf, sizeof(buf), fmt, ...) — one token added, one vulnerability closed.
• Fuzzer harnesses are production security infrastructure: they run in CI, often with broad filesystem access, and deserve the same security scrutiny as application code.
• sizeof(buffer) as the size argument is more maintainable than a literal: it survives buffer resizing without introducing a silent mismatch.
• Static analysis catches this class of bug reliably: the Orbis AppSec scanner flagged this with rule V-001 before it could be exploited, demonstrating the value of automated scanning on every commit.

How Orbis AppSec Detected This

• Source: The getpid() return value and the hardcoded format string "/tmp/libfuzzer.%d" flowing into the format argument of sprintf().
• Sink: sprintf(filename, "/tmp/libfuzzer.%d", getpid()) at fuzzer/FuzzIxml.c:45, writing into the 256-byte stack-allocated buffer filename.
• Missing control: No size argument was passed to sprintf(), providing no upper bound on the number of bytes written to filename.
• CWE: CWE-120 — Buffer Copy without Checking Size of Input ("Classic Buffer Overflow").
• Fix: Replaced sprintf() with snprintf(filename, sizeof(filename), "/tmp/libfuzzer.%d", getpid()), enforcing the buffer boundary at the call site.

Orbis AppSec automatically detected this vulnerability and opened a pull request with the fix. Try Orbis AppSec on your repositories to find and fix issues like this automatically.

Conclusion

The sprintf() → snprintf() change in fuzzer/FuzzIxml.c is one of the most well-understood fixes in C security, yet it remains one of the most commonly found issues in real codebases. The lesson here is not just about this one line: it's about establishing a habit of reaching for snprintf() by default, enforcing that habit with static analysis, and treating every piece of infrastructure code — including fuzzer harnesses — with the same security rigor as user-facing application code. A one-character change in a function name and two additional arguments are all it takes to close a class of vulnerability that has caused some of the most severe exploits in software history.

References

• CWE-120: Buffer Copy without Checking Size of Input
• CWE-121: Stack-based Buffer Overflow
• OWASP Buffer Overflow Cheat Sheet
• CERT C STR07-C: Use the bounds-checking interfaces for string manipulation
• snprintf() — Linux man page (official documentation)
• Semgrep rules for insecure sprintf usage
• fix: add buffer-length check in FuzzIxml.c